The invention relates to an electromagnetic braking arrangement, in particular for an ergometer.
European Pat. No. 0 058 718 discloses an ergometer which comprises an electric generator driven via an epicyclic gear. The generator has an exciting winding excited by external power sources and a three-phase generator winding to which external load resistances are connected. The excitation current of the exciting winding can be controlled so as to adjust the braking moment. Its possible applications are restricted by the fact that external power sources have to be available for the excitation of the generator.
U.S. Pat. No. 4 084 810 discloses an ergometer whose stator is excited by a rotating field. The rotating field generates a braking moment in an eddy current rotor, even if the rotor is driven at a comparatively slow speed of rotation. However, an external power source is also required for the operation of this known ergometer.
The U.S. patent application No. 912 592 filed on Sept. 26, 1986 (corresponding to German patent application No. P 36 31 672.5) proposes an electromagnetic braking arrangement for an ergometer, which comprises a stator and a rotor movable relative to the stator. The stator and rotor form multipole soft-iron circuits which are closed to form a magnetic circuit. One of the iron circuits has an exciting winding, whereas the other iron circuit has a generator winding and a short-circuited winding. Current from the generator winding is fed to the exciting winding and induces in the short-circuited winding currents whose magnetic field retards the relative rotation of stator and rotor. The braking moment is maintained at a predetermined desired value by a control circuit which controls the excitation current supplied by the generator winding. The control circuit is also supplied with operating voltage from the generator winding. An electromagnetic braking arrangement of this type constitutes a very compact unit and can operate without external power sources and load resistances.
In the braking arrrangement proposed in the U.S. patent application No. 912 592 the generator winding is initially excited itself by residual magnetic properties of the softiron circuit associated with the exciting winding.
The invention is directed towards provision of a control circuit for an electromagnetic braking arrangement of the type proposed in the U.S. patent application No. 912 592, which does not require external power sources for the operation of its components and can adjust the braking moment even with a very slight rate of change in the magnetic flux.